abstract object allegory of the cave aristotle bertrand russell biology charles peirce color conceptualism david kellogg lewis diogenes of sinope empiricism essence geometry george berkeley granny smith grass ideas have consequences isaac newton isagoge john stuart mill knowledge metaphysics moderate realism momentum natural kind nino cocchiarella nominalism oak objectivist epistemology peter abelard philosophical realism philosophy of mathematics plato platonic realism porphyry position pragmatism roger penrose roscellinus sound triangle truth william james william of ockham